The invention relates to a self-tapping screw with rolled thread turns, on which the angle bisector of the thread turn formed by the load flank and the rear flank slopes away from the screw head at an angle to the perpendicular to the screw axis.
A screw of this kind is known from European Patent Application 501 519. The special feature of this known screw is that both the load flank (the thread flank facing the screw head) and the rear flank (the flank opposite to the load flank) of its thread turns run into the thread root via rounded portions. This rounded configuration is presented as being advantageous in the introduction to the publication, in contrast to DE-A 32 35 352 cited therein. In the case of the subject-matter of this DE-A 32 35 352, the transition between the flank profile and the core region of the thread is of discontinuous design, this transition consisting namely of a kink. In contrast, DE-A 32 07 975, which is also cited in the European patent application, furthermore discloses a self-tapping screw where the flank angle of the thread turns increases continuously from the flank tip to the thread root, this being intended to promote damage-free deformation of the plastic in which the screw concerned is to be screwed. In addition, the European patent application points out in relation to the screw from DE-A 32 07 975 that, in the case of the screw described there, which is intended to be screwed into plastic, the thread turns are not asymmetrical, i.e. the angle bisector between the load flank and the rear flank of the thread turns is perpendicular to the screw axis.
A self-tapping screw created for use preferably with particle board is furthermore known from European Patent Application 713 017, in which the cross section of the thread turns has a kink in the region of the load flank, such that the load flank runs at a larger angle relative to the perpendicular to the screw axis after the kink from the direction of the thread tip. Moreover, in the case of this screw, the angle bisector between the load flank and the rear flank slopes towards the screw head in the region between the thread tip and the kink. As a result, the thread turns of this screw form a bearing surface that is close to the perpendicular when tightened or subjected to load, the thread turn bearing particularly heavily against material in the relevant component as the said material presses against it. As a consequence, the individual thread turns are subjected to bending stress by forces that run virtually parallel to the thread axis, considerably reducing the loading capacity of this screw.
Self-tapping screws disclosed in European Patent Application 133773 and DE-A 36 15 271 have a similar configuration since, with these known screws, the forces acting on the thread turns when a load is imposed are virtually parallel to the screw axis in the region between the thread tip and a kink in the load flank. This is particularly marked in the case of the screw disclosed in DE-A 36 15 271, in which the load flank runs perpendicular to the screw axis in the region between the thread tip and the kink. The two known screws are thus particularly sensitive when being tightened and hence to high bending loads on the thread turns.
The object on which the invention is based is to configure the screw described at the outset in such a way as to improve its production by thread rolling and furthermore to increase its load-bearing capacity and holding strength. At the same time, the geometric configuration of the screw is to be chosen in such a way as to ensure optimum screw-in performance in the respective metallic material.
According to the invention, this is accomplished by the fact that the load flank runs in a straight line to the thread root, and the rear flank initially extends in a straight line from the thread tip and, at about ⅓ of its height, merges via a kink into a straight angled portion at an angle that is greater than the angle, measured relative to the perpendicular, of the rear flank and is between 30xc2x0 and 50xc2x0, the ratio of the thread""s core diameter to the thread""s outside diameter being greater than 0.7 and the ratio of the thread pitch to the thread""s outside diameter being less than 0.25.
By virtue of the essentially rectilinear profile from the thread tips to the thread root in the region of the load flank and of the kink in the rear flank, the screw can be rolled with rolling dies whose grooves each have a corresponding rectilinear profile in cross section, a considerable advantage over rolling dies that are rounded in this area when it comes to the production of the rolling dies. Moreover, it has been found that the dimensioning that results when the ratio of the thread""s core diameter to the thread""s outside diameter is greater than 0.7 leads to a shortening of the thread turns, with the result that these are subjected to bending loads over only a relatively small radial height when the screw is tightened and when subjected to loading by the component concerned, these loads furthermore being taken in an effective manner by virtue of the fact that the angled portion extends over the region that requires firm support against buckling for the thread turns. At the same time, the fact that the kink starts at only about ⅓ of the height of the thread turns means that there is sufficient space at the outside of the thread turns to allow the displaced material from the component concerned to flow away, and this process of flowing away is therefore not impaired.
Owing to the continuously straight sloping profile of the load flank of the thread turns, the force that is exerted on this flank when the screw is tightened is introduced at a correspondingly oblique angle into the screw and hence its core because of the slope of the load flank and, because of its relatively large diameter, the core is capable of absorbing large forces. Conversely, the pressure in the material of the component is likewise directed at an oblique angle, i.e. into the material surrounding the location at which the screw is screwed in, where this pressure can be readily absorbed. At the same time, the rectilinear slope of the load flank enables uniform pressure distribution over the entire height of the thread""s load flank, thereby always allowing a maximum area of contact and maximum friction during tightening of the screw without damaging the material. By virtue of the ratio of the thread pitch to the thread""s outside diameter, which is chosen to be less than 0.25, the thread obtained has closely spaced thread turns, with the result that a relatively large number of thread turns is anchored in the component concerned over a given length. This results in correspondingly high tear-out and holding forces.
The angle between the angle bisector (between the load flank and the rear flank) and the perpendicular to the screw axis is advantageously chosen in such a way as to be between 5xc2x0 and 15xc2x0. The resulting slope of the thread turns leads to an adequate slope of the load flank, even in the case of small flank angles (measured between the load flank and the rear flank), with the result that forces acting on the load flank are introduced into the core of the screw in a favourable manner. The slope and the straight profile of the load flank are thus responsible for the friction between the screw screwed in and the component, which must be sufficiently large to ensure that the overturning torque, at which the screw would strip the thread in the component, is not reached too easily.
The screw according to the invention is particularly suitable for screwing into steel, for which purpose the screw itself is naturally likewise composed of steel. In this case, a flank angle between the load flank and the rear flank of 38xc2x0 to 48xc2x0 has proven advantageous since this gives an optimum relationship between the displaced volume and stress-bearing capacity of the material. If the steel screw according to the invention is screwed into light alloy, a flank angle between the load flank and the rear flank of 32xc2x0 to 42xc2x0 is expedient in this case.
It is also possible to make the screw from aluminium, in particular a hard aluminium alloy, which is then likewise suitable for screwing into light alloy. In this case, the selected flank angle between the load flank and the rear flank is expediently between 58xc2x0 and 68xc2x0.
The slope of the angle bisector is particularly favourable if it is chosen to be about ⅙ of the flank angle between the load flank and the rear flank.